Seasonality of oestrous cycles
At Budongo, females do not ‘come into oestrus’ (or show oestrous cycling) equally all
year round. This has been one of the more surprising things we have discovered in the
Sonso community, and the extent of seasonality was first documented by Janette Wallis,
who had worked at Gombe (Kasakela community) before coming to Budongo and so
was able to make a comparative study of sexual behaviour and oestrous cycling at the
two sites. The following is taken largely from her work on the subject (Wallis and
Reynolds 1999; Wallis 2002a).
Gombe, in Tanzania, is 4 40 south of the equator while Budongo is 1 35 north of
the equator. Though not a huge distance apart, they show mirror image seasonality and
this applies also to sexual cycles. Seasonality of sexual behaviour is more marked at
Budongo than at Gombe, with very few sexual swellings or copulations at Sonso from
May to August over the period studied (1993–1998). Onset of sexual cycles by post-
partum females occurs in the late dry season at Gombe, Mahale (Nishida 1990) and
Budongo, and adolescent females also tend to show their first swellings during this
season. At Budongo the dry season is from December to February. At Gombe the main
dry season is from May to October. When the rainfall distribution for the two sites is
plotted from January to December they look very different; when the same data are
realigned to start with the first month of the rainy season, the similarity between them
becomes apparent, although the dry season at Gombe is longer, six months as against
three months at Budongo.
Wallis compared reproductive events at the two sites by reviewing long-term data
from Gombe from 1964 to 1994 (6419 hours of observation) and from Budongo for
1990–2000 (3515 hours of observation). She looked at the presence of oestrous females
and the occurrence of sexual behaviour. At both sites, presence of oestrous females
varied seasonally. Seasonal variation was greater at Budongo than at Gombe, but both
sites showed that fewer females came into oestrus during the later part of the wet season
(see Fig. 6.5).
The same pattern occurs for resumption of postpartum cycling which most frequently
occurs during the dry season at both sites, and for copulatory activity, which declines
during the wet season and rises again towards the end of the wet season at both sites.
How are these patterns to be explained? Wallis argues that, rather than being a wet
season/dry season pattern, the differences in levels of sexual behaviour and oestrous
cycling coincide with periods of environmental change. This would help to explain the
fact that at Taï the situation appears to be the opposite from that at Gombe and Budongo,
with sexual swellings at their minimum during the dry months of June to August
(Boesch and Boesch-Achermann 2000; Anderson et al. 2002), rising during the wetter
time of year. What factors could account for such a reversal between the East African
and West African chimpanzees?
In an attempt to find an explanation, Wallis is inclined to favour the effects of
secondary plant compounds, especially plant hormones, in the diet of the chimpanzees.
This takes us into largely uncharted waters. In the forests of East and West Africa,
122 Social behaviour and relationships